In a cellular wireless system, a service area is divided into a number of coverage zones referred to as cells. Wireless terminals in a cell communicate with the base station that serves the cell. Wireless terminals may include a wide range of mobile devices including, e.g., cell phones and other mobile transmitters such as personal data assistants with wireless modems.
There are scenarios in which certain signals are transmitted from each of the wireless terminals in a cell to the base station in the cell on a regular basis. For example, a wireless terminal may be required to notify the base station of its presence in the cell at various time intervals. For a given wireless terminal the required signal transmission may not have to be precisely periodical, e.g., it may occur at a time within an assigned transmission recurring time window. One example of such regular signal transmission in a closed-loop timing controlled system is described in U.S. patent application Ser. No. 09/503,040, wherein each wireless terminal transmits a particular signal, called a timing control signal, to the base station. For each wireless terminal in such systems, the timing control signal is transmitted in regularly recurring time slots so that the base station can track the arrival time of the received timing control signal and correct the transmission timing of the wireless terminal, thereby ensuring system synchronization. However, for a given wireless terminal, the timing control signal need not, but often is, transmitted at precisely periodic recurring time instants.
Thus, one known method of scheduling the transmission of signals is to use a traditional time division multiple access (TDMA) approach, where a given wireless terminal is assigned a set of time slots that recur at precisely periodic intervals. Different wireless terminals in a cell are assigned different sets of time slots so that transmissions of those wireless terminals do not collide with each other. One drawback of this approach is that mutual interference caused by wireless terminals in adjacent cells may be highly correlated. This is because when a time slot assigned to a wireless terminal A corresponding to a first base station substantially overlaps with a time slot of another wireless terminal B corresponding to an adjacent base station, the next time slot of wireless terminal A will also overlap with the next time slot of wireless terminal B as the assigned time slots recur periodically. Correlated interference of this type causes signals transmitted by the same two wireless terminals to repeatedly interfere with each other over a long period of time. If the two interfering wireless terminals are disadvantageously located, the base stations in the overlapping cells may not be able to detect the signals correctly from the two interfering wireless terminals for a long period of time.
A problem with known cellular communications systems is that transmission by wireless devices in one cell may collide with transmissions by wireless devices in a neighboring cell. When transmissions by a device use the same frequency or set of frequencies repeatedly, multiple collisions may occur over a period of time due to the operation of devices in neighboring cells. This problem is particularly noticeable where transmissions are periodic or nearly periodic.
In view of the above discussion, it becomes apparent that there is a need for minimizing the potential for collisions between transmissions which occur in neighboring cells of a wireless communications system. In addition, it is desirable that the probability that transmissions from devices in neighboring cells will collide on a periodic basis be minimized thereby allowing increasing the chance that transmission from a device to a base station will not be blocked do to collisions for extended periods of time.